The present invention relates to a three-dimensional shape measuring method, more particularly, to a method of measuring an object having a three-dimensional shape, by projecting grid patterns upon the object and using a projection image thereof.
It is conventionally known to measure the three-dimensional shape of an object by projecting grid patterns upon the object and using a grid image thereby obtained and deformed in accordance with the height distribution of parts of the object, such methods being, for example, the moire method and the heterodyne method. In the moire method, a reference grid is superimposed upon the deformed grid image, thereby generating Moire fringes, which provide contour lines of the object to be measured, to obtain the height distribution. In the heterodyne method, using a non-modulated spatial carrier frequency signal and a carrier signal spatially phase-modulated as the reference grid and the deformed grid image, the amount of deformation is detected as a phase, thereby obtaining the height distribution of the object to be measured.
These methods are effective in the measurement of a three-dimensional shape of an object having smooth faces and parts mutually continuously connected. On the other hand, these methods are not applicable to measuring a three-dimensional shape of an object having large discontinuous steps nor an object composed of isolated faces having no mutual contact points. This is because, in the case of the moire method, a fringe order of the discontinuous contour line cannot be uniquely determined and, in the case of the heterodyne method, the height distribution of such a discontinuous object cannot be uniquely determined from the phase distribution detected in such a manner as being folded in the range of the principal value of [xe2x88x92xcfx80, xcfx80].
In order to measure the three-dimensional shape of an object which has such discontinuous steps, Unexamined Japanese Patent Publication No. 10-246612 discloses a method comprising the steps of: projecting upon the object to be measured a two-dimensional grid pattern formed by superimposing a plurality of one-dimensional grids each having a period and direction different from those of the others; imaging a two-dimensional grid image deformed in accordance with the three-dimensional shape of the object to be measured; detecting a phase for each of the one-dimensional grid components from the two-dimensional grid image; and obtaining the measurement value of the three-dimensional shape of the object to be measured on the basis of the detected phases.
In such a conventional method, however, if a larger number of one-dimensional grids having different periods and directions are used, there are more intersections among the one-dimensional grids in the grid image. In the case of an increased number of intersections existing in the grid image, frequency separation is difficult after imaging the two-dimensional grid image. In this case, the one-dimensional grids affect each other as noise sources and, therefore, precise measurement of the three-dimensional shape of the object to be measured is made impossible.
The present invention was made to solve the aforementioned problem. More particularly, the object of the present invention is to provide a method of measuring the shape of an object having three-dimensions, with improved precision.
In order to attain this object, there is provided a method for obtaining a measurement value of a three-dimensional shape of an object, using a grid image formed by projecting a plurality of grid patterns upon the object to be measured, the method comprising the steps of: projecting the grid patterns upon the object to be measured, the grid patterns comprising a plurality of one-dimensional grids of different colors, each having a distinctive period and direction; imaging the grid patterns deformed in accordance with the three-dimensional shape of the object to be measured; separating from the grid image each of the one-dimensional grids of different colors; detecting a phase for each of the one-dimensional grids; and obtaining the measurement value on the basis of the detected phases.
The one-dimensional grids of different colors may be red, green and blue. Also, it is preferable that a straight line connecting the center of a projection lens, which projects light sources in the form of grid patterns, with the center of an image formation lens, which senses the image of the grid patterns, is parallel to a reference surface on which the object to be measured is placed. Furthermore, the method may further comprise a step of measuring color information of the object to be measured by imaging the object to be measured by use of white light.